1. Field of the Invention
This invention relates to an information processing apparatus, and particularly to an information processing apparatus suitable for a magneto-optical information recording-reproducing apparatus in which a disk having a magnetic film is rotated and information signals are recorded concentric circles or in a spiral on the magnetic film by the utilization of a light beam or the signals recorded on the magnetic film are reproduced or erased.
2. Description of the Prior Art
In prior-art information processing apparatus, for example, in prior art magneto-optical information recording-reproducing apparatus, a disk which is called an magneto-optical disk comprising a thin film of ferromagnetic material formed on a nonmagnetic member such as a glass plate is generally used as the recording medium.
During the recording of information signals, the magnetic film on this recording medium is magnetized in advance in a predetermined direction perpendicular to the surface of the film and an extraneous magnetic field in the direction opposite to said predetermined direction is caused to act. Then a light beam modulated in accordance with the information is applied to the magnetic film from a recording unit comprising a laser light source, an objective lens, etc. The portion to which the light beam has been applied is increased in temperature by the light energy and, when it reaches the curie point temperature (about 160.degree. C.), the direction of magnetization becomes disordered. When the position of the light beam is then moved to another portion of the recording medium with the movement, the portion to which the light beam has so far been applied is decreased in temperature and is again magnetized in the opposite direction of magnetization by extraneous magnetic field. In this manner, the information is recorded as the reversal of magnetization on the magnetic film and a signal row is formed.
On the other hand, during the reading of information signals, a light beam oscillated from a laser light source such as a semiconductor laser, provided in the reproducing unit and polarized by a polarizer, is applied to the signal row formed on the magnetic film and, by the magnetic Kerr effect, the light beam is reflected as a light whose plane of polarization has been rotated in accordance with the direction of magnetization of the portion to which the light beam has been applied. This reflected light is separated from the incident light by a beam splitter or the like of the reproducing unit, and is directed to a light-receiving element through an analyser, and the direction of magnetization of the signal row is detected from the direction of polarization of the reflected light by a detection signal obtained from the light-receiving element, whereby the information is reproduced.
FIG. 1 of the accompanying drawings shows an example of the construction of the optical system of a magneto-optical information recording-reproducing apparatus according to the prior art. In FIG. 1, reference numeral 1 designates a semiconductor laser, reference numeral 2 denotes a collimater lens, reference numeral 3 designates a polarizer, reference numeral 4 denotes a polarized beam splitter, reference numeral 5 designates a reflecting mirror, reference numeral 6 denotes a tracking mirror, reference numeral 7 designates an objective lens, reference numeral 9 denotes a quarter wave plate, reference numeral 10 designates a condensing lens, reference numeral 11 denotes a cylindrical lens, reference numeral 12 designates an analyser, and reference numeral 13 denotes a light-receiving element. These components 1-13 together constitute a recording and reproducing unit and are placed on a head member, not shown, and may be moved as a unit radially of a recording medium 14 by drive means, not shown. The mirror 5 and the tracking mirror 6 are pivotably mounted and can vibrate at a high frequency in the direction of the arrow. Further, the objective lens 7 is of the generally used voice coil type and is drivingly controlled in the direction of focus.
In the apparatus of FIG. 1, during the recording, a light beam modulated in accordance with the information to be recorded is emitted from the semiconductor laser 1. The light beam is collimated by the collimater lens and has its direction of polarization determined by the polarizer 3, whereafter it is condensed on the recording medium 14 by the objective lens 7 via the polarizing beam splitter 4, the reflecting mirror 5 and the tracking mirror 6, and the information is recorded in the process as previously described. Although not shown, a bias magnetic field producing device for producing an extraneous magnetic field is disposed near the portion of the recording medium to which the light beam is applied. Also, at the same time, part of said light beam is reflected and returns along a part of the optical path, and is directed from the polarizing beam splitter 4 to the light-receiving element 13. At that time, this reflected light passes through an astigmatic system comprising the condensing lens 10 and the cylindrical lens 11 and is received by the four-division light-receiving element 13. Therefore, any focus deviation is detected by the light-receiving element 13 and the objective lens 10 is driven in conformity with the detection signal, whereby the distance between the objective lens 10 and the recording medium 14 is kept constant. At this time, the mirror 5 and the tracking mirror 6 may be electrically fixed and held. In this manner, the above-described optical system 1-13 constituting the recording-reproducing unit is placed on a head member (a carrier) movable radially of the recording medium 14 and is moved radially in response to the rotation of the recording medium 14, thereby recording the information circularly or spirally on the recording medium 1.
During the reproduction of the information, an unmodulated light beam emitted from the semiconductor laser 1 is applied to the recording medium 14 and any variation in the direction of polarization of the reflected light therefrom is discriminated by the analyser 12 to thereby reproduce the information. In that case, the output of the semiconductor laser 1 is made smaller than that during recording (for example, 1/2 of the output during recording), thereby suppressing the temperature rise of the magnetic thin film to prevent the recorded signals from being affected.
Now, it is usually the case with the recording medium 14 that it is eccentric, though slightly, due to the vibration or the like thereof. Therefore, reproduction of information is effected while the so-called tracking control is effected in which the amount of eccentricity of the recording medium is detected by the output signal of the light-receiving element 13 and the tracking mirror 6 is driven in accordance with the detection signal to cause the light beam to properly scan the signal row.
The magneto-optical recording apparatus of this type is capable of accomplishing very high density recording and has a recording density higher by one to two units than that of a magnetic disk. For example, a magneto-optical disk having a diameter of 300 mm is capable of containing theron about 10,000 pages of information of format A4, 16 pel/mm. An example of the then recording condition is shown in FIG. 2 of the accompanying drawings, wherein the width Dw in the major scanning direction of the recording beam spot BS is of the order of 1.3-1.8 .mu.m and the pitch P in the radial direction (the minor scanning direction) of the bit row of a recording bit BB is about two to three times the width Dw.
A futher feature of the magneto-optical disk is that it permits not only recording and reproducing of information but also re-recording and permits unnecessary information to be erased and new information to be again written into the erased portion. In the prior-art magneto-optical recording apparatus, one of the following systems is adopted as the erasing system for erasing the bit information once recorded on the magneto-optical disk:
(i) The system whereby the track (the recorded bit row) of the magneto-optical disk is scanned by a light beam used for information recording and bit information is erased on each track having a width of 1-5 .mu.m; and PA0 (ii) The system whereby an extraneous magnetic field greater than the coercive force of the magnetic film is applied by a permanent magnet or an electromagnet and the erasure of the bit information in a wide area is effected at one time. PA0 (a) In the erasing system mentioned under item (i) above, the erasure of bit information is effected in units of one track and therefore, it takes much time to effect erasure of a wide range of information. For example, assuming that the track pitch is 3.3 .mu.m and the magneto-optical disc is rotating at 1800 r.p.m., it will take ten seconds or more to erase track information of a width of 1 mm and it will take one minute or more to erase track information to a width 1 cm. Thus, this system is not suitable for high-speed erasure of information. PA0 (b) In the erasing system mentioned under item (ii) above, the magneto-optical disk rotates at a high speed of 200-1800 r.p.m. and generally, for the purpose of protecting the recording layer, the surface thereof is coated with a protective film. The magneto-optical disk has a magnetic layer in the interior in the direction of thickness thereof and therefore, there is a gap or space on the order of 1-5 mm from the surface of the magnetic field producing means (the extraneous magnetic field) to the magnetic layer. Therefore, in the surface of the magnetic layer opposed to the end portion of the magnetic field producing means, owing to that gap, there exists an intermediate range of the magnetic field intensity between a range having an erasable and a range having an unerasable non-magnetic field. Thus reliable erasure of bit information cannot be accomplished. Also, this system is characterized by the possibility of wrong erasure of necessary data owing to an unnecessarily wide permanent magnet or electromagnet and lacks reliability.
However, these conventioaal systems have suffered from the following disadvantages:
FIG. 3 of the accompanying drawings shows an example of a mode of recorded information erasure of the prior-art apparatus by the system of item (ii) above. As shown, by the magnetic field producing means 16 disposed above the recording magnetic layer 15 of the recording medium 14, a magnetic field greater than the coercive force of the magnetic layer 15 is applied to the recording magnetic layer 15 to thereby effect reversal of the magnetization of the magnetic layer 15 and to erase the recorded information. However, the magnetic field intensity of the surface of the magnetic layer 15 is considerably greater than said coercive force in a surface 17 opposed to the magnetic field producing means 16 (the portion indicated by bidirectional hatching and hereinafter referred to as the reliably erased area), but is substantially equal to said coercive force in surfaces 18 opposed to the vicinity of the end portions of the magnetic field producing means 16 (the portions indicated by hatching and hereinafter referred to as the unreliably erased areas).
In the unreliably erased areas 18, the coercive force of the magnetic layer 15, the intensity of the magnetic field produced by the magnetic field producing means 16 and the gap between the magnetic field producing means 16 and the magnetic layer 15 are irregular and therefore, it is impossible to clearly prescribe the boundary between the erased area and the non-erased area.